It is known to change the concentration of one or more gases in a liquid by passing the liquid through a chamber having a wall which is composed of a separation membrane which is substantially impermeable to the liquid and permeable to the gas(es). In one application, a liquid which is to be used in an analytical or assay procedure is “degassed” in order to reduce the concentration of dissolved gases and/or gas bubbles in the liquid. In this specification, the term “degassing” is used to include both the removal of dissolved gases and the removal of gas bubbles (which is sometimes referred to in the art as “debubbling”). The liquid can for example be an HPLC eluent or an analytical sample. Reference may be made, for example, to U.S. Pat. Nos. 3,751,879, 4,325,750 4,469,496, 5,183,486, 5,340,384, 5,876,604, 5,693,122, 5,749,942, 5,980,742, 6,248,157, 6,258,154, 6,309,444, 6,315,815, 6,494,938, 6,596,058, 6,709,492, 6,949,132, 7,022,157, 7,399,345, 7,427,312, 7,686,590 and EP 0973031. The entire disclosure of each of those patents is incorporated herein by reference for all purposes.